Secondary lithium electrochemical cells, and particularly lithium batteries, using an intercalation compound as the positive electrode, or cathode of the battery have been studied intensely during the past decade. Heretofore, the cathode material used in these batteries was typically a lithiated cobalt oxide, nickel oxide, or manganese oxide. Lithiated transition metal oxide batteries are being studied as an alternative to current nickel-cadmium and nickel-metal hydride cells because they possess several attractive characteristics, e.g., high cell voltage, long shelf life, a wide operating temperature range, and use of non-toxic materials. The earliest reports of LiNiO.sub.2 and LiCoO.sub.2 as the positive electrode materials in rechargeable lithium batteries occurred more than a decade ago and are shown in, for example, U.S. Pat. Nos. 4,302,518 and 4,357,215 to Goodenough, et al.
These materials have been intensively investigated, and one of them, LiCoO.sub.2 is currently used in commercial lithium ion batteries. Numerous patents have been issued for different improvements in these materials as the positive electrode for lithium cells. An example of a recent improvement is illustrated in U.S. Pat. No. 5,180,547 to Von Sacken for "HYDRIDES OF LITHIATED NICKEL DIOXIDE AND SECONDARY CELLS PREPARED THEREFROM". The Von Sacken reference teaches fabricating the hydroxides of lithium nickel dioxide fabricated in an atmosphere including a partial pressure of water vapor greater than about 2 torr.
Regardless of the particular material used in such cells, each material is synthesized in an oxidizing environment such as O.sub.2 or air at temperatures higher than about 700.degree. C. using nickel or cobalt and lithium containing salts. For example, a publication to Ohzuku, et al published in the Journal of the Electrochemical Society, Vol. 140, No. 7, Jul. 19, 1993, illustrates at Table 1 thereof, the typical processing methods for preparing LiNiO.sub.2. Each of the methods illustrated in the Ohzuku, et al reference show preparing the material in an oxidizing environment of either oxygen or air.
Charge and discharge of the materials fabricated according to these processes proceeds by a charge mechanism of de-intercalation and intercalation of lithium ions from and into these materials. The materials synthesized by the prior art methods have a reversible capacity of about 135 mAh/g. In other words, about 0.5 lithium ions can be reversibly deintercalated and intercalated from and into each mole of LiNiO.sub.2 or LiCoO.sub.2.
A significant amount of the capacity of these materials resides at potentials higher than about 4.2 volts versus lithium. If more than 0.5 lithium ions is removed from each of either a LiNiO.sub.2 or LiCoO.sub.2 electrode, potentials higher than 4.2 volts versus lithium are required causing decomposition of most electrolytes. Further, removal of more than 0.5 lithium ions will result in irreversible changes in the structure of these materials, causing a decrease in their capacity during charge and discharge cycles. This result was reported in a publication by Xie, et al prepared at the Electrochemical Society Fall Meeting, 1994, Extended Abstract No. 102, Miami, October 1994.
The reversible capacities of the most commonly used materials synthesized in O.sub.2 and air atmospheres are very sensitive to residual inactive lithium salts such as Li.sub.2 O, LiOH, and LiCoO.sub.3, each of which result from the synthesis process. However, to make stoichiometric LiNiO.sub.2, which is perceived to have the best performance of any of the prior art materials, excess lithium salt is normally used in precursor materials. As a result, the presence of residual lithium salt is inevitable in the final product fabricated according to prior art methods. In addition to causing a decrease in the capacity of LiNiO.sub.2, the presence of residual lithium salts often causes gas evolution such as CO.sub.2, H.sub.2 and O.sub.2 at the positive electrode during charging. Further, it is normally observed that the initial charge efficiency is much lower for LiNiO.sub.2 (i.e., less than about 80%) than that for LiCoO.sub.2 when the two materials are made in a similar fashion. In order to reduce these problems, manufacturers typically try to minimize or eliminate residual lithium salts from the product.
Accordingly, there exists a need to develop a new cathode material for rechargeable electrochemical systems, which is fabricated of materials which are relatively environmentally friendly, may be fabricated at relatively low temperatures and which demonstrate performance characteristics superior to those of the prior art. Specifically, such materials should have: (1) high capacity greater than 170 mAh/g at potentials between 3.5 and 4.2 volts; (2) an easy synthesis process which can be highly controlled; (3) insensitivity to residual lithium salts; (4) high initial charge efficiency; and (5) high reversible charge/discharge reactions so as to provide a material having good cycle life.